Development of an experimental model for high dose rate endocavitary irradiation of the rat rectum

Experimental data indicate a greater tolerance for endocavitary than for external beam irradiation of the normal rectal tissue in the rat. Increased tolerance has been attributed to temperature induced rectal hypoxia and a reduction of the dose to the mesentery. The general scarcity of experimental work in the field and the problems concerning hypoxia and dosimetry motivated the development of the current model. An 8 mm diameter endocavitary applicator was used for symmetrical dilatation and central introduction of the 192Ir source into the rectum of male Fischer F344 rats. Pulse oximetry and pO2-electrode readings from the intubated rectum gave no indication of radiobiological hypoxia. In vitro and in vivo thermoluminescence dosimetry (TLD) was supported by MRI based ferrous sulphate gel dosimetry. Using a 25 mm length source configuration the 90% isodose incorporated a 13 mm length segment of the rectum, and a 5.6% maximum deviation from the calculated dose was observed by the TLD and MRI based gel dosimetry. The ureters, the bladder and the skin were the only organs other than the target organ which received doses greater than 10% of the rectal dose. The model seems to be suitable for dose effect studies since endocavitary irradiation of the rat rectum can be performed without induction of local tissue hypoxia.